Morphogenesis of the Sternum in Quail Embryos

The flat bone develops through intramembranous ossification, in which the mesenchymal cells are directly driven towards osteogenic lineage without the formation of cartilage template. While long bone develops through endochondral ossification, where cartilage template act as an intermediate stage between mesenchymal and bone tissues. Although the avian sternum is a flat bone, some studies describe the formation of a cartilage template during its development. The aim of the current study was to observe the mechanism of ossification in quail sternum during embryonic development. Thirty quail embryos were collected for the current study (5 embryos/ day) during the period between Day (D) 5 and D10 of embryonic development and processed for light microscopy. The differentiation of mesenchymal condensation into the chondrogenic cells was observed at D5 whereas the secretion of extracellular matrix could be evident at D6. The cartilage primordia were observed by D7 which were consisted of chondrocytes, embedded in the matrix and surrounded by perichondrium. Later these primordia were developed into cartilage template by D8 where the chondrocytes were present in their lacuna. This template attained the shape of future sternum by D9, which was more distinct at D10. These preliminary observations suggested that the quail sternum grows through endochondral ossification. The future study will further explore the histological changes of quail sternum during post-hatching development.